Photovoltaic autonomy — panels and batteries
Describe the system (orientations, tilt, location, appliances) and find out how many panels and how much battery capacity you need to be independent from the grid, especially in winter. Change the panels or the battery and autonomy is recalculated instantly, by month and by season.
Why winter matters
The system is sized for the weakest month (December): if it covers winter, the rest of the year runs with a surplus.
Production depends on orientation and tilt
A south-facing array, tilted at ~30–40°, produces the most. East/West lose ~10–15%, North much more. You can have several arrays with different orientations (roof pitches).
Irradiation varies sharply by month
In December there are ~5 times fewer sun-hours than in July. That is why a system sized for summer consumption leaves the house without energy in winter.
The battery covers the night and cloudy days
Panels produce during the day; the battery stores energy for the night. For real autonomy you need 1–2 days of reserve to get through periods without sun. A depth of discharge (DoD) of ~80% protects the battery.
Shifting consumption to daytime increases autonomy
Large appliances that can be scheduled (washing machine, dishwasher, water heater, car charging) used at midday draw directly on the panels’ production and spare the battery — they increase autonomy without extra panels.
Location
Or use real location-based data (irradiance + average temperature, Open-Meteo):
Panel arrays (orientation · tilt · count)
Appliances / consumption
Power (W) × hours/day. The ☀ button marks appliances that can be shifted to daytime.
Battery
Your system
Recommended for ~100% in winter
Calculated for the orientation of the first array, in the weakest month (December). Increase the number of panels or the battery and watch the autonomy below.
Autonomy by season
Autonomy and production by month
Off-grid sizing: panels, batteries, controller
The classic stand-alone system calculation — the one you do on paper before you order anything.
The charts above show how much you cover with what you already have. The calculation here answers the reverse question: how much installed power, how much battery and which controller you need to get through the weakest month. The daily requirement is taken straight from your appliance list, so you never end up with two different figures for the same consumption.
Wp = E / (G × nsys) · Q = E × A / (V × T × ninv × ncable) · I_ctrl = 1.4 × Isc
Daily requirement (from the appliance list)
6.93 kWh/day
The daily average for the design month. If the system runs all year, that month is December; in our region you work with 1.5…2.5 hours.
From the battery datasheet, usually between 0.3 and 0.9. With lead-acid, the less deeply you discharge it, the more years it lasts.
How many overcast days in a row you expect. In Romania, four or five days is a reasonable winter assumption.
Fraction of the capacity per hour. The course asks you not to go beyond 25…30% per hour — which is why a large peak on its own forces a large battery.
Desired autonomy
Energy dictates the size: the days without sun ask for more than the power peak does. If you end up with a capacity that is hard to justify economically, the right question is not which battery you buy, but whether a generator or shifting the consumption covers the same days more cheaply.
The controller is sized at 140% of the short-circuit current of the whole array, not at the maximum power current — precisely so that shunt-type regulators, which work into a short circuit, stay safe. Oversizing is cheap next to the damage of a burnt-out controller.
1.Multiply the nine loss factors together
formula:nsys = nPV→bat × nctrl × nbat × ndist × ninv × fa × fd × ft × fdio
with your numbers:0.98 × 0.98 × 0.9 × 0.98 × 0.9 × 0.95 × 0.88 × 0.920 × 0.99
gives: 58.0 %
Every link in the chain takes its share: cable, controller, battery, distribution, inverter, ageing, soiling, temperature, diodes.
2.Find the peak power required
formula:Wp = E / (G × nsys)
with your numbers:Wp = 6930 / (2 × 0.580)
gives: 5969 Wp
3.Divide by the power of one module
formula:n = Wp / Pmax
with your numbers:5969 / 410 = 14.56
gives: 15 modules
Pmax is the peak power of one module, from the datasheet. The number of modules is always rounded UP: one module short means power short exactly in the weakest month.
4.Calculate the capacity of the battery bank
formula:Q = E × A / (V × T × ninv × ncable)
with your numbers:Q = 6930 × 2 / (48 × 0.5 × 0.9 × 0.98)
gives: 655 Ah @ 48 V
5.Compare the two autonomy criteria
formula:C_E = E × A / T C_P = P_max / r_max
with your numbers:6.93 × 4 / 0.5 = 55.4 kWh | 3 / 0.3 = 10.0 kWh
gives: 55.4 kWh
Energy dictates the size: the days without sun ask for more than the power peak does. If you end up with a capacity that is hard to justify economically, the right question is not which battery you buy, but whether a generator or shifting the consumption covers the same days more cheaply.
6.Size the charge controller
formula:I_ctrl = 1.4 × Isc × n_str
with your numbers:1.4 × 11.15 × 2
gives: 31.2 A
You start from Isc, not from Impp, so that shunt-type regulators — which work into a short circuit — stay safe.
What gets overlooked
- The design month is the one with the lowest irradiation in the operating period — December, if the system runs all year. A system sized on the annual average sits in the dark two months a year.
- The overall efficiency is not a round number someone invented: it is the product of nine factors. With the course values it comes out at around 58%, meaning that of 1 kWh produced by the modules some 580 Wh reaches the load.
- The normal daily discharge of the battery sits between 2 and 20% of its capacity. A good lead-acid battery lasts up to 4,500 cycles at 30% depth of discharge, that is some 20 years.
- In photovoltaic systems you choose batteries whose capacity is quoted at C20 or C100, because those discharge times match the duty; do not compare a C10 directly with a C100.
- At most two battery strings are connected in parallel on the same cable cross-section, and the connections are made crosswise so that they discharge evenly. Do not mix new batteries with old ones, other than when replacing a faulty one.
The formulas and the factors come from lesson 3 of the “Solar photovoltaic systems installer” specialisation course. The module efficiency at STC (0.12…0.14 for polycrystalline silicon) is used here only to estimate the module area, not in the efficiency chain — the peak power already includes it. The results are indicative; protective devices, cables and voltages remain governed by I7-2011 ch. 7.11.
☀ How to maximise autonomy
- Schedule large appliances (☀) between 10:00–15:00 in winter, when the panels produce the most — you use production directly instead of the battery.
- Avoid switching on several large loads at the same time during the night; spread them across the day.
- If winter autonomy is below 100%, first increase the number of panels (December is the limit), then the battery for cloudy days.
- In winter tilt the panels more steeply (towards 50–60°) to catch the low sun; in summer a shallower pitch is better.
Appliances that can be shifted to daytime: Washing machine, Kitchen (oven/hob)
Indicative estimate. Actual irradiation depends on the exact location, shading, weather and equipment quality; for a project use location-specific irradiation data (e.g. PVGIS) and consult an authorised installer. Electrical sizing (inverter, DC/AC protection, cable) is done separately.
Where the batteries go
Battery rooms: hydrogen, ventilation, clearances, fire rating — plus what the standard says and what it does not say about a battery in a house.
The calculation above ends with a number: so many kWh, so many Ah at the system voltage. The question that comes straight after, and that usually gets settled by eye in a box room, is where you put the bank. P 118/1-2025 has a whole subchapter on it, 2.4.19, and its entire logic starts from one thing: the gas.
A lead-acid bank on charge gives off hydrogen. Hydrogen collects at the top, at the highest point of the room, where nobody looks. Everything else follows from that — the ventilation, the distance to the luminaires, the floor that does not spark on impact, the doors that open outwards.
The threshold that holds up the whole subchapter: 4% hydrogen in air
Art. 2.4.19.7 alin. (2) states plainly what the ventilation is for: to keep the hydrogen concentration below 4% by volume, the lower explosive limit. Not “as low as possible”, but below a named, checkable threshold. Art. 2.4.19.5 alin. (1) gives the other end of the range as well: the mixture is explosive between 4% and 75% hydrogen, by volume.
The same Art. 2.4.19.7 alin. (1) puts down something many installers miss: sealed or not, all lead-acid batteries can give off explosive gases, above all while charging. The word “sealed” on the case does not cancel the ventilation.
Four categories, and everything else hangs on them
Art. 2.4.19.1 classifies batteries by exactly the criterion that counts here — how much gas escapes from electrolysis: open, or closed with a vent; sealed valve-regulated, that is the VRLA acid types, with the electrolyte fixed in a gel or in the separators between the plates; closed valve-regulated, with a recombination system of at least 95%, alkaline and acid alike; hermetically sealed, alkaline.
Getting the category right is not paperwork. The exceptions in 2.4.19.2 and the direct access without a buffer room in 2.4.19.6 are granted on the type of cell, not on the size of the bank. Write “VRLA” in the design report where you actually have closed cells with recombination and you have changed your own design regime with a stroke of the pen.
The starting rule: a dedicated room
Art. 2.4.19.2 alin. (1) is a single sentence and leaves nothing to negotiate: batteries are installed in rooms dedicated to batteries. That is where you start from. Anything else is an exception and has to be found in writing, with its scope attached.
What is worth keeping from paragraph (3) is the volume condition, because it is the one that most often fails on site: the free volume of the room must be at least 2.5 times the hourly air flow that would be needed for ventilation, calculated to SR EN IEC 62485-2. If it comes out smaller, there is nothing to negotiate — you move to organised natural or mechanical ventilation.
The figures inside the room
Art. 2.4.19.3 is the article you open with the tape measure in your hand, before you order the racks:
- At least 50 mm of clear space between rows and to the walls, for air circulation. It is not an installation tolerance, it is the path of the air.
- Every row installed has an access aisle on at least one side. Clear width: at least 1.5 times the depth of the battery, but not less than one evacuation flow width (80 cm) where the aisles are on one side only, and at least 1 m, 1.20 m recommended, where they are on both sides.
- Luminaires stay at least 1 m from the batteries and are placed only above the aisles between rows.
- The relative humidity in the room does not go above 90%, and no condensation forms.
- Sulfuric acid and potassium hydroxide do not sit together: it is forbidden, temporarily or permanently, to install them in shared rooms or on shared ventilation. Batteries made up of sealed cells are the exception.
- Batteries are normally arranged on a single level, preferably on the ground floor, on the floor or on plinths, with stability assured for the seismic zone the building sits in.
How much space the standard asks for the access aisle
The depth of the bank decides, down to a threshold. Below it, the threshold decides. Enter the depth of the rack and say on how many sides you have an access aisle:
The access aisles are
The depth dictates: 1.5 × the depth goes past the minimum threshold, so the threshold no longer adds anything.
The width that comes out is a clear width — measured between obstacles, not between axes. The 50 mm for air circulation is a requirement stated separately in the same paragraph, not a part of the width of the access aisle.
Lithium-ion: a different regime, and it is written for storage
Art. 2.4.19.4 is the only place in the subchapter where lithium-ion appears, and every one of its paragraphs speaks of battery storage spaces, not of a bank in service. The distinction counts on the day somebody waves the article at you on a phone, to impose 20 m² for a battery hung on a wall:
- With an automatic sprinkler extinguishing system to SR EN 12845: storage area limited to a maximum of 20 m², storage height to 1.80 m, zones separated by aisles at least 3.00 m wide and the state of charge of the battery kept at no more than 60%.
- With fast-response sprinklers (SR EN 12845 and SR EN 12259-13 or an equivalent regulation): still 20 m², but the storage height rises to three levels, a maximum of 4.50 m on racks or palletised, with no other goods above, and the zones may be up to 12 m high. The better sprinkler buys you height, not floor area.
- With no automatic sprinkler systems: the storage rooms or buildings are used specially and only for that purpose, and containers or enclosures placed outdoors must be non-combustible and positioned at least 6 m from other equipment, buildings, structures and stores.
- Across all the variants: smoke and carbon monoxide detection, smoke extraction systems and, as a recommendation, total flooding extinguishing to the standard covering it.
Notice what is missing from the article: no ventilation requirement for hydrogen. It follows — the classification in 2.4.19.1 is built on the gas given off by electrolysis, and lithium cells do not fall inside it. Their risk is a different one, thermal runaway, and the standard deals with it through detection, smoke extraction and distances, not through air flow.
What the room looks like, from the walls to the door
Art. 2.4.19.5 alin. (1) is the substantive article and the longest in the subchapter. What to keep from it: walls and floors of at least REI 180 class A1, with communication openings through a buffer room ventilated at positive pressure and EI₂ 90-C3 S200 doors; a floor that does not spark on impact, level, able to carry the weight of the bank, and, with liquid electrolyte, impermeable or else with racks and a sealed kerb at least 10 cm high; siting under bathrooms, laundries, WCs or other wet rooms is forbidden; a clear height of at least 2.10 m and at least 75 cm above a battery mounted on a plinth; windows protected with a grille or made of wired glass; doors opening outwards, fitted with handles or with panic bars. The joinery and its operating gear fall under Directiva 2014/34/UE — ATEX. Skip this article and you are left with the distances from alin. (4) and no room to put them in.
Ventilation does not depend on the state of the battery
Art. 2.4.19.5 alin. (1) says it in a form worth quoting as it stands in the technical report: battery rooms must be ventilated, whether the batteries are charging, discharging or at rest. Not “for as long as they are charging”. The full text is in the block above.
The air flow is not estimated by eye. Art. 2.4.19.7 alin. (3) refers to SR EN IEC 62485-2, and paragraph (4) ties the charging power to the volume of the room: the free volume must be at least 2.5 times the hourly air flow needed for ventilation. Below that, organised natural or mechanical ventilation. Paragraph (2) speaks of the battery enclosure as well, not only of the room — a closed cabinet has the same problem on a smaller scale.
Art. 2.4.19.6 also settles the practical question “do I really need a buffer room ventilated at positive pressure?”. Direct access is accepted in three situations: where it is made from outside, with measures against rain and snow, through a canopy or by setting the entrance back from the line of the walls; where the cells are of the closed type with recombination and the access is from a pass-through room or from a corridor with no process installations, where staff do not work permanently; and where the batteries are sealed.
And at home? What is clear and what is left open
In the text of the standard the subchapter is called “Conformări, instalaţii şi echipamente specifice camerelor pentru bateriile de acumulatoare” — provisions specific to battery rooms. It is written for battery rooms. A 10 kWh LiFePO4 module fixed to the hallway wall does not turn into a battery room just because it holds a lot of kWh.
What is clear from the text: the starting rule is the dedicated room, and the exceptions that do exist are written for production rooms and spaces. What is left open: the dwelling appears neither in the rule nor in the exceptions; the classification in 2.4.19.1 is built on the gas given off by electrolysis, and lithium turns up only at 2.4.19.4, and there for storage.
In practice that means the answer for a house is not pulled out of 2.4.19 by analogy, but out of the battery manufacturer’s instructions and out of the conversation with the design checker, before you drill the wall. What you can do anyway, without interpreting anything: ventilate the space, do not put the battery in a bedroom, nor under a bathroom or a laundry, mount it on a non-combustible support, leave access all round it and mark it visibly. None of that can ever be held against you.
The quotations are reproduced word for word from P 118/1-2025, approved by Ordinul MDLPA nr. 267 din 28 februarie 2025 and published in Monitorul Oficial al României, Partea I, nr. 204 și 204 bis din 10 martie 2025. Two notes on fidelity, so they do not look like transcription errors: in Art. 2.4.19.4 the numbering of the paragraphs jumps straight from (3) to (5), exactly as it appears in the text; and the 1.80 m in Art. 2.4.19.3 alin. (5) is given as an example of a solution, not as an imposed threshold.
Practise off-grid sizing
Three levels, on the formulas and the factors from the stand-alone systems lesson.
Choose the correct option
Design decisions, no calculation.
The design month
A cabin lived in all year round, with no grid connection.
Step 1: Which month do you use as the basis for the calculation?
Where the energy goes
The efficiency chain from the course: cable, controller, battery, distribution, inverter, ageing, soiling, temperature, diodes.
Step 1: Of 1 kWh produced by the modules, how much reaches the load?
What dictates the battery
A consumption of 25 kWh/day, with a peak of 10 kW when several loads start at once.
Step 1: What sets the capacity of the bank?
Calculate step by step
You get hints if you get stuck.
The peak power required
A holiday house using 3 kWh/day, in December, with 2 peak sun hours a day. The factors from the course: 0.98 · 0.98 · 0.90 · 0.98 · 0.90 · 0.95 · 0.88 · 0.92 · 0.99.
Step 1: What is the overall efficiency of the system?
The capacity of the battery bank
The same 3 kWh/day consumption, two days of storage, a 48 V system, a maximum depth of discharge of 0.5, an inverter at 0.9 and cables at 0.98.
Step 1: What capacity in ampere-hours do you need?
The charge controller
Two strings in parallel, each with modules of Isc = 11.15 A.
Step 1: What minimum current must the controller withstand?
Solve it on your own
A single final answer.
A larger stand-alone system
A consumption of 5 kWh/day, a design month with 1.8 peak sun hours, an overall efficiency of 0.58. What peak power do you need, in Wp?
The capacity forced by the peak
A peak of 6 kW, with a maximum discharge rate of 0.3 of the capacity per hour. What minimum capacity does the power criterion call for, in kWh?
A bank at 24 V
A consumption of 4 kWh/day, three days of storage, a 24 V system, a maximum depth of discharge of 0.6, inverter 0.9, cables 0.98. What capacity do you need, in Ah?
Data sources
Where the radiation data comes from
The guideline profile per region is useful as a starting point, but for a particular house it is worth using real data. Two services are queried directly from the application; the others you open yourself.
Open-Meteo — weather archive
The source behind the “Fetch real data” button — global radiation and monthly average temperatures for your coordinates, with no access key.
PVGIS — European Commission (JRC)
Estimated production for a particular tilt and orientation. Used on the photovoltaic system sizing page, where the actual plane of the roof is what counts.
Global Solar Atlas — World Bank
A map of radiation and potential, with downloadable reports per location. Good for comparing two sites before you get into the calculations.
NASA POWER
Long series of radiation, temperature and wind for any point on the globe, with an open API.
The services are public and free for non-commercial use. If one does not respond, the calculator falls back on the guideline profile per region.
Discussion
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